Grinder pump and STEP (septic tank effluent) stations have been manufactured in fiberglass and high density polyethylene (HDPE) basins for over 40 years. The most recent basin material used has been the HDPE ribbed pipe material similar to that used for storm water drainage culvert pipe.
All of these pump stations are designed to be installed in the ground with exception of a few indoor stations that may sit on the basement floor or in some cases must be installed below ground within the basement level. These stations are made of molded or welded pieces of molded sections of HDPE material. This material is used because it is easy to shape and mold, it is non-corrosive and has a very long life expectancy. The HDPE material provides for a very light weight basin that is easier and less costly to produce, ship and to install. Therefore, most of the grinder pumps and STEP pump basins require anti-flotation ballast to be added to the weight of the station to overcome the uplift forces of a wide range of potential groundwater levels.
Manufacturers of these stations have developed very detailed instructions regarding the proper amount of ballast required in size, shape and weight. These instructions are provided to installers of all ranges of capabilities to produce themselves after the product is delivered for installation. The ballast requirements are typically small quantities of less than 450 pounds containing less than 3 cubic feet of concrete material. The instructions describe a round form around the base with an outside radius of 36 inches and a height of 10.0 inches. The installer must provide such a form to manufacture this ballast ring in the field that will securely encase the shape of the pump basin.
FIG. 1, an illustration from one set of such instructions shows the shape of the form typically in an earthen formed shape.
Typical concrete used in the construction industry has a density of 150 pounds per cubic foot of volume. Uplift forces of ground water or ballast uplift forces are based on the density of water at 62.4 pounds per cubic foot. The resulting benefit of concrete is taken from the density in air value of 150 lb/cu. ft. less the density of water of 62.4 lb/cu. ft. This results in a net gain of 87.6 pounds per cubic foot.
The relatively small requirement of concrete for each station of less than 3.0 cubic feet poses a problem with regards to purchase of pre mixed or sometimes called ready-mixed concrete of uniform strength and consistency. Most concrete suppliers require a 5.0 cubic yard (135.0 cubic feet) minimum order. This then requires the coordination of installations of several pump stations at one time to meet this minimum order requirement, or coordination with other needs for concrete on the site. Concrete mixer trucks are able to get to most sites but are limited in access and can require greater care to reach a remote pump station location. Many pump installations occur in back yards with limited access that limits these trucks to reach the installation. Concrete must then be conveyed by wheel barrow or buckets adding to the labor effort.
These added costs and coordination will most likely force the installer to seek other means such as pre-mixed bag mix which requires field mixing and handling. Special care must be taken to assure proper mixing and uniform consistency of the material to provide the proper compressive strength concrete to reach the required ballast results.
Varying soil conditions and groundwater conditions make the forming of earthen forms of the precise shape and dimensions problematic. This can greatly impede the proper placement of concrete ballast that is of sufficient size and combined strength to properly secure these basins. Installers are often times faced with the challenge of how to secure the station from uplift forces during the time required for the concrete to set or cure to proper strength to complete the excavation. This can result in a great deal of extra time for manpower and machinery needed to complete this pump station installation.
Pouring a wider concrete earthen area can result in a negative gain unless additional concrete is also provided to offset the density of water forces described above. This method of installation is more costly with added concrete material needed and typically requires more time and handling of this added material.
The manufacturer's ballast computations allow for the weight of the backfill soil on top of the concrete ballast ring. This then requires the concrete to be fully cured to provide the strength to secure the pump station. This is especially critical if the station is installed under very high groundwater conditions. Uplift forces on the station can be imposed almost immediately after the station is backfilled. In such systems it is, therefore, imperative that the ballast ring be secure at the time of installation. This condition will require that the ballast ring be pre-cast onto the station prior to installation.
Many attempts at field constructed forms have been used by industrious installers. These forms can be made of wood, plywood, concrete well tiles, sections of plastic (PVC) pipe or HDPE or corrugated metal pipe have also been used. Some of these forms are made to be reused and others may be left in place. These methods can be very successful if done with care and attention to detail. They all require added time, manpower, and equipment to be completed. These measures are best taken well in advance of the actual installation and require pre-planning by the installer. Concrete typically requires a minimum of 7-14 days to reach proper strength to permit the movement of the structure. There are special add mixes and variations of concrete materials that can be used to increase the strength and reduce the set time. These materials are not commonly found at typical building supply and hardware outlets. These materials also require special knowledge and expertise to properly achieve the desired results.
Pre-casting a concrete ballast ring in controlled environments requires a means or method to then lift the entire structure of ballast and pump station and transport them to the installation location. Lifting hooks of sufficient size are required to provide strong lift points that are balanced and stable to support the pump station. Added concrete is usually required to provide support for this transport of the combined structure. FIG. 2 is an elevation view illustrating such an installation of the entire structure.
When properly made, a pre-cast ballast ring can greatly expedite the actual field installation in a wide range of soil and groundwater conditions. The pre-cast ballast ring provides the immediate advantage of securing the station as soon as the backfill soil is placed and compacted in sufficient manner to meet the manufacturer's requirements.
What is needed, therefore, are techniques for precasting of ballast rings in safe, readily installed configurations.